Thousands of people marched in central Bucharest on Sunday to honour the victims of a weekend disco fire as the death toll from the tragedy climbed to 30.

Police said some 10,000 people marched from the city's emblematic University Square to the scene of the blaze, where many were already gathered before a sea of flowers and candles set out by mourners.

Authorities said three more badly burned victims succumbed to their injuries on Sunday, bringing the death toll from the late Friday blaze to 30, adding that the toll could still rise "significantly".

"I came to pay homage to those who died and to show my support for those who are fighting for their lives," one marcher in his early 30s, Gabriel Mistodie, told AFP.

"It was a tragedy caused in a way by corruption, indifference and incompetence... and a sign that things should change in Romanian society."

Romanian media on Sunday slammed as "irresponsible" both authorities and the owners of the Colectivu nightclub where the fire and subsequent stampede killed 27 on the spot and left nearly 200 injured.

Some 140 were still in hospital on Sunday, with 35 in critical condition, according to state secretary for health Raed Arafat.

Investigators say 29 of the injured have yet to be identified.

President Klaus Iohannis had on Saturday pointed to safety failings at the nightclub, a former shoe factory.

"It is unimaginable that there could have been so many people in such a (small) space and that the tragedy happened so quickly because simple rules were ignored," he said after visiting the scene. "We already have indications that the legal regulations had not been respected."

The Evenimentul Zilei newspaper, under the headline "Coincidence, a curse or criminal negligence?", alleged that two other nightclubs belonging to one of the owners of the Colectivu club have also been destroyed by fire in recent years.

An editorial in the daily Gandul accused authorities of failing to impose stricter safety regulations in bars and discos after similar deadly accidents.

It blamed the disaster on "irresponsibility" and "unscrupulousness".

The interior ministry said between 300 and 500 mostly young people had been in the disco.

Witnesses said fireworks unleashed a blaze that was followed by a stampede as terrified clubbers scrambled to get out through only one exit and that there was no emergency exit.

Flammable soundproofing

A representative of the private company that carried out renovations in the industrial building told the DigiTV channel that the club's owners had sought to skimp on security measures to save money, and failed to consult the fire service.

He said poor quality flammable material was used for soundproofing, which caused the blaze to spread rapidly.

The disco also did not have the required authorisation to hold concerts or to stage pyrotechnic displays, Arafat said.

Investigators spent about 10 hours at the scene on Saturday to gather clues.

"We have collected samples of fabric, soundproofing material and other elements, that we must now analyse," said the head of the team of experts, George Gaman.

"We have identified those among the injured in hospital whose condition allows them to tell us what happened," the prosecutor's office said.

According to several witnesses, most of the victims were overcome by thick smoke before being trapped by the flames.

"To get out I had to dig through the bodies of those who were lying, unconscious, in front of the only exit," one of the survivors, a young man who did not give his name, told Romanian television.

The tragedy inspired gestures of solidarity across the country.

In Bucharest and in several large cities, hundreds of people queued up at transfusion centres to give blood for the injured.

Several musicians and groups also announced that all receipts from their concerts would go to help the victims and their families.